Electric current converting



April 2l, 1942. D. l. BQHN ETAI. R- 22,075.

ELECTRIC CURRENT GONVERTING SYSTEM l Original Filed Sent. 22, 1938 3 Sheets-Sheet 1 lll lll lll Ill 21, 1942. D. l. Bol-1N :TAL Re. ,22,075

ELECTRIC lCURRENT CGNVERTING SYS'IEM I original Filed sept. 22, 19:58 z sheets-Sheet 2 III III lll lll April 2.1, 1942. D. l. BOHN Erm. Re- 22.075

ELEC'JRIG CURRENT CONVERTING SYSTEM Original Filedv Sept. 22,' 1'958 5 Sheets-Sheet 3 Reuued Apr. 21, 1942 ELI-:omc

CURRENT VCONYEEIIN(Tr SYSTEM nonna r. Paasmars.. ma ouun'n x. Marti; Wnwatosa, assigner: to

All!! Wis.. Chalmers Manufacturing Company, Milwaukee, Wis., a corporationV of Delaware original-No. 2,166,900, ama July 1s, 193s, seranA No. 231,094, .September 22, 1938. Application for reissue October 9, 1940, Serial No. 360,514

'Ihis invention relates in general to improvementsvin electric current converting systems of the electric valve type, and more particularly to means for controlling the flow of alternating current components of different frequencies in the connections of a converting system Joiningan alternating `current circuit with a direct current circuit.

Electric current converting systems comprising electric valves, which are frequently utilized forv transmitting current between a polyphase alternating current circuit and a direct current circuit, have `the inherent property of causing the flow of alternating current components of different frequencies in the connections thereof with the associated circuits, While these current components are frequently harmless, they may induce disturbing currents in communication circuits adjacent alternating current circuits connected with large converting systems of the type considered. Y i

For the purpose of reducing the distributing `effect of these .current components, converting systems have been devised which are Aintended to inherently prevent the ilow of the` larger and most disturbing of lthe current components. As is well known, this result is usually sought by connecting the valves of the system with the alternating current circuit through a suitable phase multiplying transformer. For example,

alternating current rectifying systems frequently comprise a phase doubling transformer having a. three phase primary winding and a six phase cuits may, on the contrary, have considerable magnitudes therein.

Instead of using only one phase multiplying transformer having a secondary winding comprising a large number of phase portions, the system may comprise a plurality o'f transformers having secondary windings so connected that each transformer secondary winding forms a polyphase system of a relatively low number of phases and that all the secondary windings jointly form a polyphase system ofa relatively large number of phases. For example, a phase doubling transformer having a star connected primary winding and a second phase doubling transformer having a polygon. connected primary winding may be used in parallel to jointly multiply the number of phases of the system to four times the number of phases of the alternating current circuit. To further increase the number of phases of the system, it is, however, again necessary to resort to fork or zigzag connected windings divded into'unequal sections, which only permit partly to reduce the magnitudes of the particular current components of which the flow should be substantially prevented in the connections of the system.

These disadvantages of phase multiplying transformers Amay be avoided by utilizing a plurality of transformers of which each leaves the number of phases of the system unaltered or increases the number of phases of the system to twice the number of phases of the alternating current circuit, and by increasing the number of secondary winding. In transformers of this type,

the windings may be star connected windings or polygon connected windings having each phase portion thereof inductively related with only one of the legs of the associated core. The windings may therefore be symmetrically arranged tof cause the voltages thereof to form a well balanced polyphase system of voltages. When it is desired to increase further the number of transformer phases, however, it becomes necessary to utilize so-called zigzag or fork connected windings, in which each phase portion vis divided into a plurality of unequal sections associated with different phase legs of the core. The transformer then becomes excessively complicated and expensive to manufacture, and it has been found by experience that it is not`possib1e to obtain therefrom an accurately balanced polyphase system of voltages at all loads. As a'result of this unbalance, the current components which should be inherently suppressed in the connections of the converting system with'the associated cir-l phases of the system to a number greater than four times the number of phases of the alternating current circuit by connecting the transformers with the alternating current circuit through suitable phase shifting means; The desired suppression of certain current components in the connections of the system may also be obtainedv i It is therefore one of the objects of the pres-n ent invention to provide an electric current converting system of the electric valve type in whichj certain current `components may be substantially suppressed in the connections of the sys- Atem.

Another object of the present invention is to provide an electric current converting system of the electric valve type in which the number 0f phases of the system may be made equal to more than four times the number of phases of 'an alternating current circuitl connected V therewith v a first star connected winding I8 divided into three equal phase portions and a second star through a plurality of phase vdoubling transferniers.

Another object of the present invention is to provide an electric current converting system of the electric valve type in which the number of phases of the system may be made-equal to more than four times the number of phases of an alternating current circuit connected therewith by means of phase Shifters distinct from the usual hase doubling transformers. f p Another object of the present invention is to provide an electric' current converting system of the electric valve type in which the flow of certain current components in the connections of the the cons stem may be controlled by controlling dictivity of some of the valves of the system.

Objects and advantages other than those above l described will be apparent from the following description when read in connection with the acin drawings, in which: cogimlv dagrammaticall'y illustrates one em@ bodiment of the present invention in which fivel parallel six phase conifrters are connected to hirt hase sys m; y fogilg'; diagrmmatically illustrates aumodif'led embodiment of-the present invention in winch four parallel six phase converters are connected to form a. twenty-four phase system;

F'ig. 3 is a diagram of the currents in some of \the connections of the embodiment illustrated in Fig. l with the associated alternating current is a diagram of the currents flowing in some of the connections of the embodiment illus trated in Fig. 1 when one of the converters is and mamisea diagram of the currents in some of the connections of the embodiment. illustrated in Fig. 2 with the associated alternating curren circuit.

Referring more particularly to the drawings by characters of reference, reference numeral 6 designates a polyphase alternating current circuit' may comprise any predetermined number of onductors, a three phase -circuit is herein iliasstted for the reason that this type of circuit is most frequently utilized in practice. Afst a matter of convenience, circuit G will'hereina te be considered as being a supply circuit connec e) with a suitable current generator (not shownh and circuit 1 as being an output circuit, althoug the system is also'operable for the transmissionl of energy between a direct current supply circui and an alternating current output circuit. l

The diilerent converters are arranged' similar to one another and a detailed description of a the converters vis therefore believed unnecessaryf. Converter 8 may be taken as representative of all the convertersy and comprises a plurality o electric valves generally designated by I3.. Theis: valves are severally provided with anodes which are preferably arranged within a conmon casing, the cathodes of the valves then smeg combined into a common cathode structure f Suitable means (not shown) are provided or bringing cathode I6 into electron emitting conr-1 dition and for maintaining the cathode in suc `on, as is well known. coxiiilles Il. are severally'connected with cirl rality of single phase structures.

co ected winding I3 divided into six equal pha `portions. Transformer I1, like all other transformers to be described hereinafter, may be a unitary Vstructure of the polyphase shell type or core type, orv may consist of an equivalent plu- Under tli'eassumptionsA made above, winding I8. which is connected with circuit (8, is the primary winding of transformer Iland winding IB, which is connected with those valves of the system designated by I3, is the secondary Vwinding of the transformer. Winding I8 is preferably provided with a plurality of taps to permit adjustment of the magnitudes of the terminal voltages of winding I9. The'designations of star connected winding and polygon connected winding are herein intended to apply only to windings in which each phase portion, i. e., each portion joining two terminals or joining a terminal with a neutral point is inductively related with only one leg of the associated core. The phase porti'ons Ioi winding I9 are preferably divided into two groups to provide two neutral points severally connected with the negative conductor of circuit cuit 6 through a main transformer l1 having 75 1 through an interphase transformer 2l. L Cathode I6 is connected with the positiveconductor of circuit 1 to complete ,the path of the iiow of current through the converter. The connections between converter 3 and circuit 1, as well las the portion of circuit 1 carrying current received from converter 8, are advantageously arranged to be endowed 'with a material amount of inductance which may be supplemented by the insertion therein of an inductive--reactor 22.

The conductivity of valves I3 may be con-` trolled by means comprising control electrodes 23 severally associated with anodes Il and each arranged in or about the path of the discharge between' the associated anode and cathode I6.

,Although valves I3 may bek of any suitable known type, it may be assumed that the valves are of 'the discontinuously controllable type. Each control electrodelaccordingly prevents the ilow of current through the associated anode when the control electrode is at a predetermined negative potential with respect to the potential of cathode I6 taken as datum. The control electrode also releases the flow of current through the associated anode upon being brought from a negative potential to a positive potential, but cannot regulate the flow of current. To obtain this result, the different control electrodes are preferably severally connected with cathode I6 through circuits each comprising a current limiting resistor 24 and one of the phase portions of the secondary winding of a control :transformer 26 having the primary winding thereof energized from circuit 6.' The secondary winding of transformer 26 is star connected to provide a neutral point connected with cathode I6 through a source vof variable unidirectional voltage such 'as a battery 21. Circuit 1 supplies current to current consuming devices of any kind such as electrolytic cells 28 preferably arranged in a plurality of parallel banks of serially connected cells.

While converter I2 comprises a main transformer 32 connected in the same manner as the primary windings of transformers I1 and 29 to 32 were all connected directly with the conductors of circuit C, as was heretofore the usuai practice, the thirty secondary winding portions.

of the transformers would jointly form a bal" ancd polyphase system having only twelve different phases, i. e., a number of different phases.

not exceeding four times the number of phases of circuiti. To cause the voltages ofall the secondary windings to form jointly a polyphase System of voltages having a number of phases greater than four times the number of phases of circuit 9, transformers Il and 29 tov 32 areconnected with circuit 9 through means comprising suitable phase shifting means such as autotransformers Il to 3i.V

Phase shlfters 33 .and 3G are similar to one are assumed to be of the usual sinusoidal wave form. The current flowing between the corresponding conductor of circuit i and a converter,

another in every particular and are both arranged to introduce a displacement of six de-l ing direction, and the voltages of transformers I1 and 3l are shifted in the lagging direction.

The vwinding phase portions of the different main transformers have been oriented in the drawings toindicate the phase relation that obtains between the voltages thereof. It will -be' Transformers I1 and 29 to 32. are preferably so arranged as to have substantially equal reactances. Phase Shifters 33 to 3B are likewise so arranged as to have substantially equal re The reactances of converters 9, 9, ll

actances. and I2 are therefore substantially equal while that of converter lil has a slightly lower value. Converters 9, 9, Il and I2 accordingly tend to supply equal currents to circuit l while converter lll tends to supply a greater amount of current to circuit 1. A suitable reactance means such as a polyphase reactor 31 is preferably inserted in the connections of transformer 39 with circuit 6 to substantially equalize the reactances of all the converters and thus cause all the converters to deliver equal currents.

To obtain a uniform control of the conductivity of the valves of the different converters, the different control transformers thereof, of which only transformer 26. is illustrated in the draw-- sented by curve 4I'.

such as converters l and l2, assumed to be directly connected therewith, would Vbe substantially of the rectangular wave form repre- TheV current flowing between the same conductor of circuit I and a converter such as converters 9, .Il and ll assumed directly connected therewith, would be substantially v of the stepped wave form represented by curve 42, as is well known. It can be shown that these currents each comprise a fundamental component having the same frequency as the voltage of circuit l and a plurality of harmonic components' of dierent orders. These har- `monic components are' sinusoidal alternating components of vdifferent frequencies multiple' of vthe fundamental frequency of the voltage of circuit 9, the order of any harmonic being the ratio of the frequency of the harmonic to the fundamental frequency. Forexample, an analysis of both curve 4l and curve 42 would reveal that the most prominent harmonic components thereof oare of the orders 5, '7, 11, 13, 17, 19, 23, '25, 29 Aand 31, and are of decreasing magnitudes in the order recited.

If curve 39 is assumed to represent any of the primary voltages. to neutral of transformers' Il and 29 .to 32, regardless of the phase relationship therebetween, curve 4I also represents any of the currents owing in the' primary winding connections of transformers I1 and 32, and curve 42 also represents any-of the currents flowing in the primary winding connections of transformers 29, 30, and 2|. 'I'he joint current flowing through circuit B to the converting system is not, however, the sum of currents represented by curves 4I and 42. If the currents flowing from circuit 9 to the different converters were of sinusoidal wave form, the joint current 'flowing through circuit 6 would also be of sinusoidal wave vform and would then be equal. to the sum of the several currents flowing in the transformer connections regardless of the presence of phase Shifters 33 to 39, because the phase shifters do not disturb the angular relation between a sinusoidal current or current component and the corresponding sinusoidal voltage of the same ings, are preferably-connected with circuit 9 in vsuch manner Cthat the phase relationship between the control electrode potentials and the potentials lof the associated anodes is the same in all converters.` For example, the control transformers of converters 3', 9, Il .and I2 may be connected with circuit 6 through suitable phase shifters such as autotransformer 3B and three further autotransformers (not shown).

frequency. The phase shifters, however, cause the harmonic components of the currents to be displaced in phase with respect to the fundamental components thereof by. different amounts in the currents of the vdifferent phase Shifters, and the Wave forms of the currents flowing between circuit 6 andthe phase Shifters are no longer those of curves 4I and 42. The currents flowing between circuit 6 and phase Shifters 33, 34,/ 35 and 36 are then represented by curves 43 to 46, respectively, while the current flowing between circuit 6 and .transformer 3|) of converter Ill is represented by a curve 41 duplicate of curve 42.

For the sake of simplicity, the different control autotransformers are preferably all of a unitary design provided with a plurality of taps to permit proper adjustment of the phaseshifts vimparted thereby to the voltages impressed on the different control transformers.'

To render more apparent the advantages of the system described above, reference is made to Fig. 3 in which curve 39 represents. one of the voltages to neutral of circuit 9, which voltages The joint current flowing between circuit 6 and the different converters,l whichis also the current in circuit 6 if no further current-consuming or converting devices are connected therewith, is the sum of the currents represented by curves 43 to 4l, and is represented by a curve 48. As a result of the connection of the converters in the manner described above, the sum of the harmonic components of the eight lowest orders in all the currents supplied by circuit 6 to the converters is equal to zero and these particular components are therefore compensated in the joint current 4.7 v l flowing ln circuit 5.- Of the components remaining in the current of circuit 8 thelargest component is the order 29. These remaining com- 22,o75 Y flowing through circuit 5 even fwhen the secoiidary voltages of transformers I1\and 29 to 32 do not form a Vcompletepolyphase system of ponents are quite harmless by reason both of v their low magnitudes and ofthe relatively'reduced effect of currents of the corresponding frequencies in communication circuits. In fact, 'it has been found that the reactance opposed by the alternating current circuit'to the ow of these current components is generally sufficiently high to limit the magnitudes thereof to a negligible value. The current flowingthrough circuit 6 is then practically of sinusoidal wave form.

Because the secondary voltages of transformers l1 and 29 to 32, which are impressed through the valves on circuit 1, form a polyphase system, the output voltages of the different converters vary continuously and are generally different from one another; the converter having momentarliy the highest output voltage tends accordingly to momentarily supply the greater part of the current transmitted by circuit 1 to cells 28. Each of the different converters thus tends to carry a fluctuating current comprising a substantially uniform current component supplied to the cells and a circulating current component flowing through circuit 1y to the terminals of the other converters. 'I'he reactors, such as 22, or the equivalent inductive connections between the `convertersVincluding the conductors of circuit 1, serve to control the flow of this circulating current between the converters. By suitablyselecting the arrangement `of the conductors of circuit 1 and the connections thereof with the converters and, if necessary, by adding suitable reactors in these connections, the flow of circulating current between the converters Ymay be reduced to any desired extent.

While the system described above is capable of completely preventingthe flow of harmonic current components of the eight lowest orders in circuit E when the voltages of circuit 5 are accurately balanced and of perfectly sihusoidal wave form, it has been found that residual harmonic components of the eight lowest orders may flow through circuit as a result of the presence of harmonic components in the voltages of this circuit. It is,'however,V possible to reduce the magnitude of these residual components to a great extent by altering the conductivity of the different valves of the-system. This result may be" obtained either by displacing the taps of some of the control batteries, such as battery 21, or by shifting the phase of some of the voltages impressed on the control transformers by disl 41 only.

voltages. This operatingV condition may` obtain, for example, if one ofthe converters is temporarily disconnected from the system to permit inspection and eventual overhaul yof'any of its elements. If it is assumed, for instance, that converter I2 is rendered inoperative, the current flowing through circuit 'becomes represented by curve 50 of Fig. 4 having ordinates equal to.

the sum of the ordinates of curves 43, 44, 45 and In thiscurrent the harmonic components of orders lower than the 29th are present, although the magnitudes thereof are smaller than they would be in theabsence Vof phase Shifters 33, 34 and 35. 1 These harmonics may be substantially cornpensated in the current of circuit 6 by delaying the flow of current throughk the valves of some of the converters to the extent necessary to cause the different currents owing through all the remaining valves, or at leastthe fundamental components of these currents, to form substantially a complete polyphase system of currents. In the present instance, the periods of current flow through valves f3 preferably remain unaltered, and the periods of current `flow through the valves of converters 9, In and Il are delayed by 3, 6 and 9 electrical degrees respectively. As is well known. the periods of current flow may be so delayed by adjustment of the taps o f the control batteries or of the control phase Shifters. The fundamental components of the main transformer secondary currents then form a complete polyphase system of currents having twenty-four phases in which successive currents are relatively displaced by fifteen electrical degrees instead of being lrelatively displaced by twelve degrees as circuits of all the valves may be jointly readis the original thirty phase system.

When the periods of current flow through the valves are delayed by means of the valve control electrodes, the wave form of the currents flowing through the main transformers remain substanf tially unaltered. `The currents owing between circuit 6 and the different converters likewise substantially retain their original wave form and may be represented by curves 5I, 52, 53, and 54 which are duplicates of curves 45, 41, 44, and 43, respectively, curves 5I, 52 and 53 respectively, being, however shifted in phase by 9, 6' and 3 electrical degrees lagging relatively .to curve 54. The joint current flowing through circuit 6 is then represented by curve 56 obtained by adding the ordinates of curves 5I to 54. The most prominent harmonic component of this current is now of the 23rd order, and all other harmonic components are of higher orders.

The adjustment disclosed above was described neglecting the fact that the output voltages of converters 9, IU and I I are lowered to a slight extent by 'delaying the periods of current :dow through the valves'thereof and that the currents supplied from circuit '6 to the four remaining converters are therefore slightly unequal. These currents may, however, be maintained at exactly equal values by readjusting the taps of the primary windings of transformers 29, 30 and 3|.

The converters then also deliver currents of slightly different magnitudes to circuit 1, and

valves oi' the remaining three converters may be v readJusted in a similar manner to cause the fundamental components ofthe main transformer secondary currents to form an system of currents.

It may occasionally' also become necessary to operate the system without autotransformers Il to 3l, for example, if the autotransformers have to be rewound in previsionfof a change in the value of the voltage of circuit l or of circuit 'l.v

system of currents.. The magnitudes" of the cor-A responding currents in the connections of the "converters with circuit I may then be adjusted to exactly equal values by readjusting the taps of the main transformer primary windings. The latter currents retain substantially the wave forms of curves 4i and l2 but are mutually dis- I placed by twelve electrical degrees. The current flowing through circuit I, which is the sum of the converter currents, may be shown to again have The system illustratedin Fig. l may be modified y ln different manners to form systems in which the number of phases is increased by different amounts 4beyond the number of vphases of circuit 6. For example, by providing transformers i1 and 32 with polygon connected primary windings and transformers 28, 3|) and Il with star connected primary windings, a system maybe formed having the same properties as the system illustrated. The modified system and the system illustrated may also be connected in parallel to form jointly a system involving a number of diil'erent phases I.

equal to sixty, Aor twenty times the number of phases of circuit l.

The system may also be modified as illustrated in Fig. 2, in'which the converters illustrated in Fig. 1 are again utilized with the exception of converter Ill. One-half ofthe converters, coneighteen phase .r rectly connected with circuit 6.

supplied by circuit i to converters l and I through winding portions thereof and form a well balanced polyphase system of voltages having a number of phases equal to twenty-four, or eight times the number of phases of circuiti.

The wave forms of the different currents flowing through circuitiv and the dierent converters of the present embodiment are illustrated in Pig.

v5. Curves 4| and 42 reproduced therein without change to represent the current flowing between circuit i and converters I2 and il, re-y spectively, inasmuch as these converters are di- The currents phase shifter 49 are represented by curves l1 and 58. respectively. As in the embodiment i1- lustrated in Fig. l, the harmonic components of the different converter currents are displaced in phase with respect to the fundamental components by different amounts in the'difi'erent currents. 'I'he sum of the six harmonic components of the six lowest orders of the four converter currents equals zero. These harmonie components are therefore -compensated in the Joint current-flowing in circuit l, which is equal to the sum of the converter currents as represented by curve 59. The remaining harmonic components are usually reduced to a negligible value by the inductance of circuit 8.

In the vpresent embodiment also it is assumed that the connections between the converters through circuit 1 are sufhciently inductive to reduce the flow of circulatingl currents between the converters to a negligible value. If the connections between the converters and the conductors ofcircuit 'l are relatively short, the reactances thereof may be relatively small compared to the reactances of the diiferent portions of the conductors of circuit 1 joining the several banks of cells 28 and which may be lassumed to be concentrated in a plurality of inductive reactors 6I. These reactors control the flow of circulating currents between the different converters to the l same extent as reactors 22 illustrated in Fig. 1.

sisting of converters l I and l2, are connected with circuit 8, either directly, as shown, or through a 'polyphase reactor similar to reactor 31 illustrated in Fig. 1. Converters l and I constituting the other half of the converters are jointly connected with circuit through .a single polyphase phase shifter illustrated as an autotransformer 49 in which the output voltages are displaced .by fifteen degrees relatively to the input voltages. It will -be understood that an equivalent result'would be obtained by connecting both halves of the converting system with circuit B through any phase shifter giving a relative phase displacement of fifteen degrees to the two system halves or through two separate phase Shifters giving the same result. Regardless of which connection is selected, the secondary voltages of transformers i1, 29, 3| and 32 have the relative phase displacements indicated by the orientation 0f the Although but two embodiments of the present invention have been illustrated and described, it

will be vapparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. In an electric current converting system, the -combination with a polyphase alternating current circuit of a predetermined number oi' phases, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having'a iirst winding connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a polyphase system having a number of different phases not exceeding four times the first said number'of phases, of means connecting said first windings with said alternating current circuit to cause the voltages of all said second windings to form a polyphase system of voltages having a numberof` phases greater than four times the first said number of phases. 4

2. In an electric current converting system, the

combination with a polyphase alternating current circuit of a, predetermined number of phases.

'a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves sacra first windings and said alternating current cirin the Joint current flowing in said alternatingV current circuit.

3. In an' electric current converting system,

the combination with a, polyphase alternating.

current circuit oi a predetermined number of phases, a direct current circuit, a plurality of yparallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a lirst winding connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a polyphase system having a `number of different phases not exceedingnfour. times the first said number of phases whereby the currents flowing between the said rst windings and said alternating current circuit each comprise a fundamental component and a plurality of harmonic components of different orders, of means connecting said iirst windings with said alternating current circuit to cause the said harmonic components to be displaced in phase with respect to the saidv fundamental components by different amounts in the different said currents whereby the sum of said harmonic components of at least the four lowest ones of said orders is caused to equal zero.

4. In an electric current .converting system, the combination with a polyphase alternating current circuit, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprlsinga plurality of electric valves and a transformer having a first winding connected with said alternating 'current circuit and a second winding connected with said valves, said second windings jointly forming aVV system having a predetermined number of dif- 'A ferentV phases, of means comprising phase shifting means connectingv said rst windings with said alternating current circuit to cause the voltages of all said second windings to vform a polyphase system of voltages having a number of phases greater than! the iirst said number of phases. 1

5. In an electric current converting system, the combination with a polyphase alternating current circuit of .al predetermined number of phases,

aY direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a rst winding connected with said alternating current circuit and a second winding connected withsaid valves, said second windings jointly forming a polyphase system having a number of diiferent phases not exceeding four times the iirst said number of phases, of

means comprising a phase shifting autotransformer connecting Vat least one of said iirst windings with said alternatingcurrent circuit to cause the voltage of all said second windings to form a polyphase system of voltages having a number of phases greater man four times the erst said number of phases.

6. In an electric current converting system, the combination with a polyphase alternating current circuit of a predetermined number of phases, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a flrst connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a polyphase system having a number of diierent phases not-exceeding four times the first said number of phases, of means comprising phase 'shifting means connecting certain ones-only of said iirst windings Ywith said alternating current circuit to cause thevoltages of all said second windings to form a polyphase system oi voltages having a number of phases greater than four times the first said number of phases, and reactance means 'connesting the remainder of said transformers with said alternating current circuit to substantially equalize the reactances of all said converters.

7. In an electric current converting system, the

combination with a polyphase alternating current circuit of a predetermined number of phases, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a ilrst winding connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a polyphase system having a number of different phases not exceeding four times the first said number of phases, of

. circuit of a predetermined number of phases, a f

direct current circuit, a plurality of parallel current'converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a ilrst winding connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a polyphase system having a number of different phases not 'exceeding four times the first said number of phases, of means comprising a plurality of phase Shiftersl severally connecting all of said transformers, except one, with said alternating current circuit to cause the voltages of all said sec- Y ond windings to form a polyphase system of voltages having a number of phases greater than four times the firstsaid number of phases. y

9. In an electric current converting system, the combination with a polyphase alternating currentl circuit of a predetermined number of phases, a direct current circuit, a plurality of parallel current converters interconnectingv said circuits and each comprising a plurality of electric valves and a transformer having a first winding connected with said alternating current circuitand a second winding'connected with said valves, said'second windings jointly forming a polyphase system having a number of dierent phases not exceeding four times the first said number of p hases, of means'comprising' a plurality of phase Shifters severally connecting all of amers' said transformers, except one. with said alternating current circuit-to cause the voltages of all said sec nd windings to form a polyphase systemoi vol es having a number of phases greater than four times the first said number of phases, and a polyphase reactor connecting the said one oi' said transformers with said alternating cur'- rent circuit to substantially equalize the reactances of all said converters.

10. In an electric current converting system, the combination with a polyphase alternating current circuit of a predetermined number of phases, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a first winding connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a polyphase system having a number of different phases not exceeding four times the rst said numberof phases, of means connecting said first windings with said alternating current circuit to cause the voltages of all said ,second windings to form a polyphase system of'voltages having a number of phases greater than four times the first said number of phases, and inductive means in said direct current circuit vfor controlling the now of circulating current between said converters.

1l. In an electric current converting system, the combination with a polyphase alternating current circuit of a predetermined number of phases, a direct current circuit, a plurality of parallel current `converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a first winding connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a polyphase system having a number of different phases not exceeding four times the rst said number of phases, of means comprising a plurality of phase Shifters severally connecting all of said transformers, except one, with said alternating current circuit to cause the voltages of all said second windings to form a polyphase system of voltages having a number of phasesgreater than four times the first said lnumber of phases, a polyphase reactor connecting the' said one of said transformers with said alternating current circuit to substantially equalize the reactances of all said converters, and inductive means in said current circuit of a predetermined number oi phases, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality ot electric valves and atransformer having a iirst winding connected with said alternating current circuit and a second winding connected with said valves, certain ones of said first windings -being the combination with a polyphase alternating current circuitof a predetermined number of n phases, a direct current circuit, yfour parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a first winding connected with said alternating current circuit and a second winding connected with said valves, two of the said first windings being connected in polygon connection and all the other ones of said windings being connected in star connection, of phase shifting means connecting one of said polygon connected first windings and one of said star connected first windings with said alternating current circuit to cause the voltages of said second windings to form a polyphase system of voltages having a number of phases equal to eight times the first lsaid number of phases.

15. In an electric current converting system, the combination with a polyphase alternating current circuit of a predetermined number of phases, a direct current circuit, five parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and said windings being connected in star connection, of phase shifting means connecting two of said polygon connected first windings and two direct current circuit for controlling'the flow of t circulating current between said converters.

'12. In an electric current converting system, the combination with a. polyphase alternating current circuit of a predetermined number of phases, a direct current circuit, a plurality of parallel current converters yinterconnecting said circuits and each comprising a plurality of electric valves and a transformer having a first winding cnnected with said alternating current circuit and a second winding connected with said valves, at least certain ones of said windings being connected in star connection and the remainder of said windings being connected in polygon connection, of means connecting said first windings with said alternating current 'circuit to cause the voltages of all said second windings A'to form a polyphase system of voltages having a number of phases greater than four times the first said number of Yphases.

13. In an electric current converting system, the combination with a polyphase alternating 'the combination with a polyphase alternating current circuit of a predetermined number of phases, a direct current circuit, five parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a first winding connected with said alternating current circuit and a second lwinding connected with said valves, at least two of the said first windings being connected in polygon connection and all the other ones of said windings being connected in star connecvtion, lof phase shifting means connecting two of said polygon connected first windings and two of said star connected first windings with 'said alternating current circuit to cause the voltages of said second windings to form a polyphase sys- Atem of voltages having a number of phases equal to ten' times the mst said number or phases.. and reactance means connecting the remaining one of said rst windings with said alternating cur- Vrent circuit to substantially equalize the reactances of all said converters.

17.1In'an electric current converting system. the combination with a polyphase alternating current circuit of a predetermined number vof phases,v a direct current circuit, five parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and arv transformer having `ailrst winding connected with said alternating' current circuit and a second vwinding connected with said valves, at leastv two of the said first windings being oonnected in polygon connection and all the other ones of said windings being connected in star connection, of phase shifting means connecting two of said polygon connected first windings and two of said star connected first windings with said alternating current circuit to cause the voltages of said second windings to form a polyphase system of voltages having anumber of phases equal to ten times the first said number ofphases, reactancel means connecting the remaining one of said first windings with said alternating current circuit to substantially equalizethe reactances of all said converters, and inductive means in said direct current circuit for controlling the iiow of circulating current between said converters.

't 18. In an electric current converting system, the combination with a polyphase alternating current circuit of `a predetermined number of phases, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each' comprising a plurality of elec- -tric valves and a transformer having a first winding connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a polyphase system having a number of different phases not exceeding four times the first said number of phases whereby the currents flowing between the said first windings and said alternating current circuit each comprise a fundamental component and a plurality of harmonic components of different orders, of means connecting said first windings with said alternating current circuit to cause said harmonic components of at le'ast the four lowest ones of said orders to be. compensated in the joint current iiowing in said alternating current circuit, and means for controlling the conductivity of said valves for adjusting the first said currents to compensate residual harmonic components in the joint current flowing in said kalternating current circuit.

19. In an electric current converting system, the combination with a polyphase alternating current circuit, a direct current circuit, a plurality of parallel current converters interconnecting said Ycircuits and each comprising a plurality of elec'- tric valves and a transformer having a first winding connectedwith said alternating current circuit and a second winding connected with said valves and supplying currents thereto having fundamental components, oi means connecting saidL rst windings with said alternating current circuit to cause said fundamental components to form'jointly an incomplete polyphase system of currents, and means for controlling the conductivity of said valves to cause said fundamental components to be relatively displaced lin phase to form a complete polyphase system of currents.

20. In an electric current converting system. the combination -with a polyphase alternating current circuit, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and .each comprising a plurality of electric valvesand a .transformer having a first winding connected with said alternating current circuit and a second winding connected with said valves and supplying currents thereto rhaving fundamental components jointly forming nected primaries and star connected secondaries whereby'phase potentials of the alternate transformers areshifted in phase with respect to each other, and an auxiliary transformer connected in vseries with one of said rectifier groups for fur' ther shifting the phase of the potential applied to one group of rectifers.

22. An electric current converting system comprising four parallel converters each including a fi-phase transformer and a plurality of electric valves, two of said transformers being connected to impress on the valves associated therewith a 12-phase system of voltages, and phase shifting means connected in series with the other two of said transformers to cause the voltages impressed' on the valves associated therewith to form a 12-phase system of voltages displaced by l5 electrical degrees from the rst said 12-phase system.

23. In an'electric current converting system,

l the combination with a polyphase alternating current circuit, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a first winding connected with` said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a system having a predetermined number of different phases, of means serially connected with said transformers comprising phase shifting means for causing the voltages impressed on said valves from said transformers to form a polyphase system having a number of phases greater than the first said number of phases.

24. In an electric current converting svstem, the combination with a polyphase alternating currentcircuit, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a first winding connected with said alternating current circuit and a second winding connected with said valves, said second windings jointly forming a system having a predetermined numbe: of different phases, of means comprising a phase shifter serially connected with one of said transformers for causing the voltages impressed on said valves from said transformer to form a. polyphase systern having a number of phases greater than the first said number of phases.

25.Inv an electric current converting system, the combination with a polyphase alternating current circuit oi a predetermined number of phases, a direct current circuit, a plurality of parallel current converters interconnecting said circuits and each comprising a plurality of electric valves and a transformer having a winding connected with said alternating current;I circuit and a winding connected with said valves, at least certain of said windings being connected in star connection and the remainder of said windings being connected in polyphase connection, of means serially connected with at least one of said windings for causing the voltages impressed on said valves from said transformers to form a polyphase system having a number of phases greater than four times the rst said number of phases.

26. 'In an electric current converting system, the combination with a polyphase alternating current circuit, a direct current circuit, a plural'- ity of parallel current converters interconnecting said circuits and each comprising a transformer and a plurality of electric valves, and a phase shifter serially connected with one of said transformers to vary the phase of the voltages impressed from said one of said transformers on the associated ones of said valves.

' DONALD I. BOHN.

' OTHMAR K. MARTI. 

